1. Application Field of the Invention
This invention generally pertains to a piezoelectric acceleration sensor and a piezoelectric acceleration sensor device which use a film piezoelectric element. Specifically, the piezoelectric acceleration sensor and sensor device employ a simplified structure, which generates a small output by an acceleration perpendicular to the sensing axis, and offers excellent temperature characteristics and high impact resistance.
2. Prior Art
A conventional piezoelectric acceleration sensor (hereinafter referred to as "the sensor") in the public domain is presented in Published Japanese Patent Application 56-10258. This sensor has a disc-shaped diaphragm made of a piezoelectric material (such as piezoelectric polymer) which is fixed on a ring-shaped frame at its circumference. Both sides of the center of the diaphragm have loading elements which function as an inertial mass respectively. The frame is fixed on a pedestal with the diaphragm suspended.
In the sensor, an axis crosses perpendicularly to the film face of the diaphragm and runs through the center of the loading element, which forms the acceleration sensing axis. Such a sensor can detect an acceleration change in the sensing axis direction when a material to be measured is placed on the pedestal.
However, this sensor presents a problem: the loading element produces a directional displacement even when an acceleration is given in the direction perpendicular to the sensing axis. This causes distortion in the diaphragm to generate an electric output.
The sensor also has other drawbacks: its structure is complex and its manufacture is difficult. Also, its measurable frequency band is narrow and difficult to change.
A conventional piezoelectric acceleration sensor device (hereinafter referred to as the sensor device) is known publicly, which accommodates the sensor described in the above Published Japanese Patent Application 56-10258. This sensor device is basically a circuit substrate mounted with two components: an impedance conversion circuit which converts an output from the sensor, and an amplification circuit which amplifies the output. These are arranged side by side in a enclosure. However, this sensor's inevitably large size is undesirable.